RFR: notify: rename MailingListUpdater to MailingListNotifier

[Erik Helin]

Hi all,

please review this refactoring that renames `MailingListUpdater` to `MailingListNotifier`. More refactorings will
follow after this one for the other notifiers.

Testing:
- [x] `make test` passes on Linux x64

Thanks,
Erik

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Commit messages:
 - notify: rename MailingListUpdater to MailingListNotifier

Changes: https://git.openjdk.java.net/skara/pull/646/files
 Webrev: https://webrevs.openjdk.java.net/skara/646/webrev.00
  Stats: 3092 lines in 10 files changed: 1547 ins; 1544 del; 1 mod
  Patch: https://git.openjdk.java.net/skara/pull/646.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/646/head:pull/646

PR: https://git.openjdk.java.net/skara/pull/646